Don't pass "--xhtml" to hightlight in gitweb.perl script.

Don't pass "--xhtml" to hightlight in gitweb.perl script.

The "--xhtml" option is supported only in highlight < 3.0. There is no option
to enforce (X)HTML output format compatible with both highlight < 3.0 and
highlight >= 3.0. However default output format is HTML so we don't need to
explicitly specify it.

Signed-off-by: Adam Tkac <redacted>
Helped-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>