binman: Clean up unnecessary code related to ELF test files

We use the Makefile for all ELF test files now, so drop all the code that
checks whether to get the test file from the Makefile or from the git
repo.

Also add a comment to the Makefile indicating that it is run from binman.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/test/Makefile b/tools/binman/test/Makefile
index 593bbe9..bdbb009 100644
--- a/tools/binman/test/Makefile
+++ b/tools/binman/test/Makefile
@@ -1,5 +1,5 @@
 #
-# Builds test programs
+# Builds test programs. This is launched from elf_test.BuildElfTestFiles()
 #
 # Copyright (C) 2017 Google, Inc
 # Written by Simon Glass <sjg@chromium.org>