Remove reference to gdb_dwarf_index from the freebsd-clang mkspec.

Follow-up to 9de2853a

 ("Remove automated generation of dwarf index"):
gdb_dwarf_index.prf does not exist anymore, so stop referencing it.

Change-Id: I22464d5b81a50a2f58218d74a424f3a790aa1df0
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>

mkspecs/unsupported/freebsd-clang/qmake.conf

@@ -4,7 +4,6 @@
 
 MAKEFILE_GENERATOR      = UNIX
 QMAKE_PLATFORM          = freebsd bsd
-CONFIG                 += gdb_dwarf_index
 
 QMAKE_CFLAGS_THREAD     = -pthread -D_THREAD_SAFE