Simplify a .mailmap entry

The old entry mapped the email address `<compnerd@compnerd.org>` to user name
`Saleem Abdulrasool` and email address `<compnerd@compnerd.org>`. Since the two
addresses are identical, that's a needless detail.

The new entry just maps email address `<compnerd@compnerd.org>` to user name
`Saleem Abdulrasool`.

No behavior change.

Differential Revision: https://reviews.llvm.org/D108079