build: Remove po-gram-gen2.h.
* gettext-tools/src/po-lex.c: Include po-gram-gen.h instead of po-gram-gen2.h.
* gettext-tools/src/po-lex.h: Update comment.
* gettext-tools/src/Makefile.am (noinst_HEADERS): Remove po-gram-gen2.h.
(po-gram-gen2.h): Remove rule.
(BUILT_SOURCES, MAINTAINERCLEANFILES, EXTRA_DIST): Don't add it.
(po-lex.o, po-lex.lo): Don't depend on it.
