Remove sourcepath argument from metalava invocations

This is no longer required to find package.html files, so remove it.

Bug: 153703940
Test: m checkapi
Merged-In: I80ec219cb9ef8922094336a5d45652c8e59aa113
Change-Id: I80ec219cb9ef8922094336a5d45652c8e59aa113
(cherry picked from commit 556e8149)