Remove jdiff_enabled from platform metalava runs

The official API differences report is not produced from the
artifacts this rule creates, but by manually running metalava
with the right txt files.

As there are no other known users of these jdiff files, remove them
for now.

Bug: 158465496
Test: m
Change-Id: Iee9dd1a5d5543f3f629e3631c8224e6bfe4dddcb