assert: remove unreachable code

[Trott]

In lib/internal/assert/assertion_error.js, line 391 assures that
operator is 'deepEqual' so there is no need to check the value of
operator in a ternary on the next line (line 392). Remove the ternary.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines