Remove compiler support for Raw String Literals from JDK 12

Summary

With the withdrawal of JEP 326: Raw String Literals (Preview) from JDK 12, code introduced into the Java compiler to support the feature must be removed. (See the withdrawal notice and rationale.)

Problem

The Java compiler in JDK 12 supports Raw String Literals as a preview feature, but the Java language in Java SE 12 does not support them in any way.

Solution

Undo all changesets which implement the specification of Raw String Literals given by JDK-8206982.

Specification

The Java language in Java SE 12 accepts the same kinds of literal as the Java language in Java SE 11 (JLS 3.10). Consequently, the Java compiler in JDK 12 will support the same kinds of literal as the Java compiler in JDK 11.

The previously proposed language changes in the raw-string-literals attachment are reverted.