[llvm-readobj][NFC] Remove unused prefix from FileCheck tests

This is to enable --allow-unused-duplicates=false. This prefix appears
to be outdated and intentionally unused.

Reviewed By: rupprecht

Differential Revision: https://reviews.llvm.org/D90427

llvm/test/tools/llvm-readobj/ELF/needed-libs.test

@@ -58,7 +58,7 @@ ProgramHeaders:
 ## Check what we print when the dynamic string table is empty.
 # RUN: yaml2obj %s --docnum=2 -o %t2
 # RUN: llvm-readobj --needed-libs %t2 2>&1 | \
-# RUN:   FileCheck %s -DFILE=%t2 --implicit-check-not=warning: --check-prefixes=EMPTY-DYNSTR,EMPTY-DYNSTR-LLVM
+# RUN:   FileCheck %s -DFILE=%t2 --implicit-check-not=warning: --check-prefixes=EMPTY-DYNSTR
 # RUN: llvm-readelf --needed-libs %t2 2>&1 | \
 # RUN:   FileCheck %s -DFILE=%t2 --implicit-check-not=warning: --check-prefix=EMPTY-DYNSTR