w32: Fix gnupg_unsetenv.

* common/sysutils.c (gnupg_unsetenv): Don't use nonstandard extension
of "NAME", but "NAME=".

Microsoft implementation of putenv works to remove an environment
variable by "NAME=".

POSIX doesn't say that putenv with "NAME=" has same effect. GNU
implementation doesn't support this way for removal of environment
variable.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>