Remove .rustc ELF section from mini-debug-info.

The section is stripped from binaries (but preserved in /symbols).

However, is it also unintentionally copied into the minidebuginfo
compressed section, which is included in the final shipped binary.
(This does not affect symtab symbols, which are handled as usual)

Removing it from mini-debug-info shrinks some rust binaries by
up to 10x, saving >10MB on the system partition overall (AOSP).

Test: extract minidebuginfo and check it manually with readelf
Change-Id: I462d81455626bb2b5244e1bf3d5f8b71646f3401