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
Merged-In: I34164b633bab2892e3a1c0fd0257fcf98d537655
Change-Id: I34164b633bab2892e3a1c0fd0257fcf98d537655
(cherry picked from commit edd61643)