Remove temporary hack from gl2ext.h header.

hwui/debug no longer references glMultiDrawElementsBaseVertexOES after
ag/3831170 has been merged, so the temporary addition of that misnamed
function to the header is no longer necessary. The gl2ext.h header now
exactly matches the Khronos version.

Bug: 76152545
Test: Flashed Pixel XL and ran some VR test cases from CTS.
Change-Id: If4a5a2b5f1a73624bf6c96bd0b4cd4e9182a0499