Don't do a half-hearted recompilation check in compileOne

The isNothing maybe_old_linkable check predates
48bc81ad, which fixed #481 by requiring
recompilation information to be passed in as an argument to compileOne.
As a result, the check here is redundant: the client has already taken
a look at the object file to see if it is available or not.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

(cherry picked from commit af4d9980)

compiler/main/DriverPipeline.hs

@@ -171,7 +171,7 @@ compileOne' m_tc_result mHscMessage
    -- -fforce-recomp should also work with --make
    let force_recomp = gopt Opt_ForceRecomp dflags
        source_modified
-         | force_recomp || isNothing maybe_old_linkable = SourceModified
+         | force_recomp = SourceModified
          | otherwise = source_modified0
        object_filename = ml_obj_file location