Remove unused string resources

Commit 6396c47c removed
res/values/mimes.xml but the translations were left in
place, causing aapt warnings.

Test:
m DocumentsUI, observe build output no longer generates:
    warn: removing resource com.android.documentsui:string/*
    without required default value.

Change-Id: I12040bb42246703d1b979350a5df12b0ddffef39