The only reason it's included there is for a declaration of
struct sway_server, but we can just forward-declare it.
This avoids rebuilding almost all of Sway when touching server.h.
All other server.h includes are from source files, not headers.
active_keyboard may be NULL, in which case an invalid pointer could be
passed to wlr_input_method_keyboard_grab_v2_send_modifiers. This
procedure call is unnecessary since wlroots commit 372a52ec "input
method: send modifiers in set_keyboard", so the call can simply be
removed.
Fixes#6836.
Before this commit, there would be cases where focus changes from one
window to another, the new window activates text_input, then the old
window sends a deactivate request, making text_input unfocused
completely.