Remove proofread files that aren't created

These would cause these actions (and any that depended upon them) to
re-run during every incremental build. These now produce warnings:

ninja: output file missing after successful execution: out/soong/.intermediates/frameworks/base/ds-ref-navtree-docs/android_common/ds-ref-navtree-docs-proofrerad.txt

It turns out that the -proofread argument to doclava is ignored with the
following argument combinations:

 * -static -devsite
 * -navtreeonly

Test: m docs; m docs (nothing to do)
Change-Id: I21b939951f5a2f33914e9fc9414df8206091d262