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This removes the recently introduced `mojom_cpp_typemap` GN template in
favor of simply inlining typemap configs directly within the `mojom`
target they affect.

A few motivations here:

- The separate target approach required traits sources to always belong
  to a different target than the mojom variant's own sources. This has
  insurmountable dependency issues in some corner cases like
  //content/common.
- A typemap can only realistically affect a single mojom target in the
  build under most circumstances.
- A significant portion of the variables within a typemap target were
  only used by forwarding them to the affected mojom target. Inlining
  everything makes the direct impact on the mojom target more clear.
- This approach is exactly as powerful (from the perspective of build
  expressiveness) as typemap files, but without the typemap files.

Bug: 1059389
Change-Id: I8218312374e86a0c02dcaea630b05f61c7aa6d61
Tbr: changwan@chromium.org
Tbr: haraken@chromium.org
Tbr: dcheng@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2106887
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751449}