This has proven intrusive, especially when you have files from different
projects open and you switch between them rapidly. Enabling
treemacs-follow-mode is left up to the user. Otherwise, the SPC o P
keybind and treemacs-find-file command can this manually and on-demand.
Also remove hide-mode-line-mode from treemacs because it's redundant
with the doom-themes treemacs icon theme.
Alexander-Miller/treemacs#212 introduced treemacs-persist-file, which we
now use instead of tools/treemacs' old treemacs-persistence hack (which
didn't work).
Relevant to #669
