[clang][test] Remove check that fails if SOURCE_DATE_EPOCH is set glo…

…bally

The check for "no SOURCE_DATE_EPOCH" wasn't especially interesting, and
I am not aware of a _portable_ way to unset and environment variable in
a lit test. So remove it since it can fail if the build environment has
SOURCE_DATE_EPOCH set globally.

Differential Revision: https://reviews.llvm.org/D142511

clang/test/Driver/SOURCE_DATE_EPOCH.c

@@ -1,5 +1,2 @@
-// RUN: %clang -E %s -### 2>&1 | FileCheck %s -check-prefix=NO_EPOCH
-// NO_EPOCH-NOT: "-source-date-epoch"
-
 // RUN: env SOURCE_DATE_EPOCH=123 %clang -E %s -### 2>&1 | FileCheck %s
 // CHECK: "-source-date-epoch" "123"