Move NetHttpCoverageTest to external/cronet TEST_MAPPING

It seems that aosp/3086726 landed before aosp/2910846 and did not result
in any merge conflict which led to a leftover entry for cronet tests.

This CL removes it and in the same topic, it is moved to external/cronet

Bug: 338358650
Test: TH
Change-Id: If875f252aaadedff8d5ed520bdb152874b6fa9c3
1 file changed