pgsql: Don't call SetEnvironmentVariable() when removing an environment

Log Message:
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Don't call SetEnvironmentVariable() when removing an environment variable,
as this seems to crash on at least some versions of MingW. Our current usage
of this function does not require it, so it should be ok to ignore.

Modified Files:
--------------
pgsql/src/port:
win32env.c (r1.1 -> r1.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/port/win32env.c?r1=1.1&r2=1.2)