[lld][ELF] remove empty SyntheticSections from inputSections

Change removeUnusedSyntheticSections() to actually remove empty
SyntheticSections in inputSections.

In addition to doing what removeUnusedSyntheticSections() was meant
to do, this will also make the shuffle-sections tests, which shuffles
inputSections, less sensitive to empty Synthetic Sections that
will not appear in the final image.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D106427

Change-Id: I589eaf596472161a4395fb658aea0fad73318088