distro-configs: disable firebird on linux_clang_dbgutil_64

As it's quite unstable and this way the clang tinderbox fails before
deteching interesting problems (and firebird is covered by other
platforms anyway).

Change-Id: Ieaca3a6f5d036c9df22bdbe5a48bf27d2d9ce49f
Reviewed-on: https://gerrit.libreoffice.org/28223Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>

distro-configs/Jenkins/linux_clang_dbgutil_64

 --enable-dbgutil
 --enable-compiler-plugins
+--disable-firebird-sdbc