pgsql: Remove sema.c, superseded by win32_sema.c.

Log Message:
-----------
Remove sema.c, superseded by win32_sema.c.

Modified Files:
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pgsql/src/backend/port/win32:
Makefile (r1.6 -> r1.7)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/port/win32/Makefile.diff?r1=1.6&r2=1.7)

Removed Files:
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pgsql/src/backend/port/win32:
sema.c
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/port/win32/sema.c)