build: Don't hint to --with-installed-csharp-dll when C# support is disabled.
When --disable-csharp was specified, gettext-tools/src/Makefile.am does not
produce msgunfmt.net.exe. Therefore it is irrelevant whether the condition
USE_INSTALLED_CSHARP_DLL evaluates to true or false. Thus the message
"WARNING: When building the gettext-tools package without building the entire
gettext package, you need to pass the --with-installed-csharp-dll option to
configure."
is pointless.
Reported by Ryan Schmidt in <https://savannah.gnu.org/bugs/?58668>.
* gettext-tools/configure.ac: Don't emit hint to use --with-installed-csharp-dll
when BUILDCSHARP is not 'yes'.
