Revert "Windows: teach getenv to do a case-sensitive search"

Revert "Windows: teach getenv to do a case-sensitive search"

This reverts commit df599e9612788b728ce43a03159b85f1fe624d6a.

As of 5e9637c6 "i18n: add infrastructure for translating Git with gettext",
eval_gettext uses MinGW envsubst.exe instead of git-sh-i18n--envsubst.exe
for variable substitution. This breaks git-submodule.sh messages and tests,
as envsubst.exe doesn't support case-sensitive environment lookup (the same
is true for almost everything on Windows, including MSys and Cygwin tools).

30a615ac "Windows/i18n: rename $path to prevent clashes with $PATH" renames
the conflicting variable in git-submodule.sh, so that it works on Windows
(i.e. with case-insensitive environment, regardless of the toolset).

Revert to the documented behaviour of case-insensitive environment on
Windows.

Signed-off-by: Karsten Blees <blees@dcon.de>