syntax-check: drop prohibit_undesirable_word_seq

This checks forbids using "can not" and checks the placement
of some texinfo tags.

Drop it since we do not use texinfo and the check takes almost
twice as much as the rest of the checks.

cfg.mk

@@ -64,6 +64,7 @@ local-checks-to-skip =			\
   sc_prohibit_quote_without_use		\
   sc_prohibit_quotearg_without_use	\
   sc_prohibit_stat_st_blocks		\
+  sc_prohibit_undesirable_word_seq	\
   sc_root_tests				\
   sc_space_tab				\
   sc_sun_os_names			\