Cleanup: Remove NativesCollection<.>::*Raw* methods.

These methods for used for compressed libraries, where GetSource* functions
contained the compressed sources and [GS]etRawSource* the uncompressed
sources. This is dead code since the API no longer supports compression.

(If you need/want compressed sources, use the external startup data and
 compress/uncompress on the Embedder's side.)

BUG=

Review URL: https://codereview.chromium.org/772853003

Cr-Commit-Position: refs/heads/master@{#25666}