selectrum: remove automatic initialisms for M-x etc

Doesn't work if we want to use selectrum highlighting optimizations, see
raxod502/selectrum#539.

Also document `SPC /` being slow.

modules/completion/selectrum/README.org

@@ -150,9 +150,9 @@ A wgrep buffer can be opened from swiper with =C-c C-e=.
 ** Orderless filtering
 When using orderless to filter through candidates, the default behaviour is for
 each space separated inputs to match the candidate as a regular expression or
-literally. In ~command~ and ~symbol~ category searches such as =SPC h f= and =SPC
-h v=, matching by initialism is also on by default. You can further specify each
-space separated input in the following ways:
+literally.
+
+You can further specify each space separated input in the following ways:
 | Input            | Description                                |
 |------------------+--------------------------------------------|
 | =!foo= or =foo!= | match without literal input =foo=          |

modules/completion/selectrum/TODO.org

@@ -38,13 +38,9 @@ enter, it opens the new buffer in another window rather than the main one, even
 though at least the bookmark function ostensibly uses
 ~pop-to-buffer-same-window~. Ivy gets the window switched in the bookmarks and
 grep case due to a custom ivy occur window switching function.
-** TODO ~orderless+initialism~ doesn't work for some reason
-See https://github.com/oantolin/orderless/issues/54.
-Narrowed it down to the fact that it works if ~selectrum-mode~ is off, but
-doesn't if it's on (even without the ~use-package~ configuration).
-However, it has no problem working with selectrum with a MWE using ~emacs -Q~,
-so it might be caused by some nontrivial doomism in the area.
-
+** TODO selectrum =SPC /= is much slower than ivy =SPC /=
+requires further investigation. is ~consult-ripgrep~ slower than ~counsel-rg~?
+is it something the custom search function is doing? does ivy cache stuff?
 * PROJ Missing Features
 ** TODO Icons
 https://github.com/minad/marginalia/issues/59

modules/completion/selectrum/config.el

@@ -65,16 +65,10 @@
      ;; Flex matching
      ((string-prefix-p "~" pattern) `(orderless-flex . ,(substring pattern 1)))
      ((string-suffix-p "~" pattern) `(orderless-flex . ,(substring pattern 0 -1)))))
-  (orderless-define-completion-style orderless+initialism
-    (orderless-matching-styles '(orderless-initialism
-                                 orderless-literal
-                                 orderless-regexp)))
   (setq completion-styles '(orderless)
         completion-category-defaults nil
         ;; note that despite override in the name orderless can still be used in find-file etc.
-        completion-category-overrides '((file (styles . (partial-completion)))
-                                        (command (styles orderless+initialism))
-                                        (symbol (styles orderless+initialism)))
+        completion-category-overrides '((file (styles . (partial-completion))))
         orderless-style-dispatchers '(+selectrum-orderless-dispatch)
         orderless-component-separator "[ &]"
         selectrum-refine-candidates-function #'orderless-filter