Don't export flags from SourceProvider variants

There is no need to export the linkdirs from the SourceProvider
variant.  Remove them to reduce differences in build.ninja from
a later patch that moves the flag exporting into compile(), which
isn't called by the SourceProvider variant.

Test: m checkbuild
Change-Id: I9c4d3a336a07cb9074376303bfa277c05d893b98