Revert "Merge pull request #1194 from ascent12/meson_feature"

This breaks wlroots when used as a meson subproject.

This reverts commit dea311992e, reversing
changes made to 6db9c4b746.

include/wlr/backend/meson.build

@@ -1,16 +0,0 @@
-install_headers(
-	'drm.h',
-	'headless.h',
-	'interface.h',
-	'libinput.h',
-	'multi.h',
-	'session.h',
-	'wayland.h',
-	subdir: 'wlr/backend',
-)
-
-if conf_data.get('WLR_HAS_X11_BACKEND', false)
-	install_headers('x11.h', subdir: 'wlr/backend')
-endif
-
-subdir('session')

include/wlr/backend/session/meson.build

@@ -1 +0,0 @@
-install_headers('interface.h', subdir: 'wlr/backend/session')