Remove references to three deleted files.

The gyp-to-ninja conversion process doesn't check for missing files, so
they creep in. This change deletes three files that were deleted or
never added but which are referenced from .gyp files.

To see warnings about deleted files (Windows only) set GYP_GENERATORS
to msvs-ninja,ninja before running build\gyp_pdfium. The msvs-ninja
generator warns on missing files.

R=tsepez@chromium.org
BUG=483424

Review URL: https://codereview.chromium.org/1201273006.
3 files changed