Revert "Formal parameter names can be the same as function name. Fixes gh-1928"

This reverts commit 248e40bc39c1110ca7eb1d9799804ce005b9d2f5.

The issue which motivated this change related to the modification of the
environment record of ES6 concise methods. That issue was addressed in a
more comprehensive and concise manner via commit
b95f669dbb62158355c0fb6a7638c7ccf036dc93 and verified with an equivalent
unit test.