Makefile: Do not use OLD_ICONV on MINGW anymore

We are building libiconv now the same way as upstream MinGW does, so we do
not need OLD_ICONV anymore when compiling Git either in msysGit or
mingwGitDevEnv.
Signed-off-by: NSebastian Schuberth <sschuberth@gmail.com>

config.mak.uname

@@ -481,7 +481,6 @@ ifneq (,$(findstring MINGW,$(uname_S)))
 	NO_FNMATCH = YesPlease
 	NO_MEMMEM = YesPlease
 	NEEDS_LIBICONV = YesPlease
-	OLD_ICONV = YesPlease
 	NO_STRTOUMAX = YesPlease
 	NO_MKDTEMP = YesPlease
 	NO_MKSTEMPS = YesPlease