[test] Fix incorrect use of string variable use

LLVM test CodeGen/AArch64/machine-outliner-retaddr-sign-thunk.ll uses
a string substitution block that contains a regex matching block. This
seems like as a copy/paste from other similar test where the match also
defines a variable, hence the [[]] syntax. In this case however this is
a CHECK-NOT variable so nothing should match. No variable definition is
thus expected and the square brackets can be dropped.

Reviewed By: chill

Differential Revision: https://reviews.llvm.org/D98853