[compiler] Remove broken support for undetectable strings.

[compiler] Remove broken support for undetectable strings.

Support for undetectable strings was officially dropped in
https://codereview.chromium.org/916753002, but the compilers
weren't fixed properly.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1287983002

Cr-Commit-Position: refs/heads/master@{#30132}