Stop filtering html files from droidstubs input

Metalava now expects html files to be specified explicitly. This
filtering was originally added because jdiff could not cope with html
inputs, but jdiff support has been removed in r.android.com/1455335.

Bug: 153703940
Test: m checkapi
Change-Id: I34164b633bab2892e3a1c0fd0257fcf98d537655