Remove nearly all custom behavior with consult.
The separate groups for project and non-project buffers within
consult-buffer weren't as useful after switching away from orderless to
fussy for completion scoring, as non-project buffers were often sorted
higher than project buffers.
And there's no need for the custom siren-consult-projectile-buffer
function, as consult-project-buffer does the same thing when configured
to use projectile's root path function.