Remove StringUTF16::equals

JDK-8215017 removed the only use of StringUTF16::equals in favor of simply calling StringLatin1::equals to compare the byte arrays, since it turned out to be a performance win over calling into potentially specialized subroutines.

This remove a theoretical performance optimization opportunity in that the UTF16 code path can elide code to check any trailing byte. This is leveraged in the aarch64 intrinsic. Performance evaluation on x86 and aarch64 shows that consolidating to only use the more generic StringLatin1::equals intrinsic is net neutral or even a net win.

This RFE suggest cleaning out the StringUTF16::equals code and associated intrinsic support.