buildsys: Remove X_SERVER from AM_CPPFLAGS

It is a leftover from multi-seat-x wrapper which is long
gone.

Makefile.am

@@ -203,7 +203,6 @@ AM_CPPFLAGS = \
 	-DSYSTEM_SLEEP_PATH=\"$(systemsleepdir)\" \
 	-DSYSTEMD_KBD_MODEL_MAP=\"$(pkgdatadir)/kbd-model-map\" \
 	-DSYSTEMD_LANGUAGE_FALLBACK_MAP=\"$(pkgdatadir)/language-fallback-map\" \
-	-DX_SERVER=\"$(bindir)/X\" \
 	-DUDEVLIBEXECDIR=\"$(udevlibexecdir)\" \
 	-DPOLKIT_AGENT_BINARY_PATH=\"$(bindir)/pkttyagent\" \
 	-DQUOTACHECK=\"$(QUOTACHECK)\" \