Remove all .gyp and .gypi files.

Except:
* //build/linux/system.gyp
* //third_party/boringssl/boringssl.gypi
* //skia/skia_gn_files.gypi
(which are all currently consumed via //build/gypi_to_gn.py).

Also remove the remaining contents of //build/linux/unbundle.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/1438853002 .