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

.mailmap

@@ -32,4 +32,4 @@ Jon Roelofs <jonathan_roelofs@apple.com> <jonathan@codesourcery.com>
 Jon Roelofs <jonathan_roelofs@apple.com> <jroelofs@jroelofs.com>
 LLVM GN Syncbot <llvmgnsyncbot@gmail.com>
 Martin Storsjö <martin@martin.st>
-Saleem Abdulrasool <compnerd@compnerd.org> <compnerd@compnerd.org>
+Saleem Abdulrasool <compnerd@compnerd.org>