This patch removes parser code implementing desugaring for ESnext
public and private fields on classes. The desugaring should probably
be implemented in the interpreter instead, and more work needs to go
into optimization and debugger support. The actual parsing of class
fields is left in, as the syntax is relatively stable, and there are
strong cctests for the grammar.

R=marja
BUG=v8:5367

Review-Url: https://codereview.chromium.org/2578893005
Cr-Commit-Position: refs/heads/master@{#41776}