[Test] Remove unnecessary PYTHONIOENCODING

PYTHONIOENCODING is already specified in `lit.cfg`. The change that
added this also added the `<`, which is what actually made this pass -
PYTHONIOENCODING only overrides the encoding of stdin/stdout/stderr.

Author: bnbarham

test/ScanDependencies/unicode_filename.swift

@@ -2,7 +2,7 @@
 // RUN: %target-swift-frontend -scan-dependencies %/s %/S/Inputs/unicode_filёnamё.swift -o %t/deps.json
 
 // Check the contents of the JSON output
-// RUN: env PYTHONIOENCODING=UTF-8 %validate-json < %t/deps.json
+// RUN: %validate-json < %t/deps.json
 // RUN: %FileCheck %s < %t/deps.json
 
 public func bar() {