Revert "boilerplate.py: Ignore current-dir prefixes .\, ./"

This reverts commit 9d14b9ec.

Reason for revert: Change is breaking calls without .\ prefix.

Original change's description:
> boilerplate.py: Ignore current-dir prefixes .\, ./
> 
> This change makes boilerplate.py trim a leading ".\" (Windows's
> current-directory prefix) or "./" (Unix's current-directory prefix)
> from the filename argument, if present.
> 
> This is a convenience change for cases where the current-directory
> prefix is prepended. For example, Windows PowerShell automatically
> prepends ".\" to a path when tab completion is used (e.g., typing
> "chrome/b" and pressing tab results in ".\chrome\browser"). This
> change will remove artifacts of this for users who use tab completion
> while typing the filename argument.
> 
> Currently:
> `tools/boilerplate.py .\test.h` produces include guard `__TEST_H_`
> `tools/boilerplate.py ./test.cc` or `.mm` produces `#include "./test.h"`
> 
> With this change:
> `tools/boilerplate.py .\test.h` produces include guard `TEST_H_`
> `tools/boilerplate.py ./test.cc` or `.mm` produces `#include "test.h"`
> 
> Change-Id: I19fd2e72c58135b5e87d4a1e22ec8bc76ef326aa
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2157876
> Commit-Queue: Jesse McKenna <jessemckenna@google.com>
> Reviewed-by: Robert Sesek <rsesek@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#761085}

TBR=rsesek@chromium.org,jessemckenna@google.com

Change-Id: Ic02537133fd1548f5279ead83abb0492d6657aaa
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2159993Reviewed-by: Jesse McKenna <jessemckenna@google.com>
Commit-Queue: Jesse McKenna <jessemckenna@google.com>
Cr-Commit-Position: refs/heads/master@{#761185}

tools/boilerplate.py

@@ -55,15 +55,6 @@ def _CppHeader(filename):
   ])
 
 
-def _RemoveCurrentDirectoryPrefix(filename):
-  current_dir_prefixes = [os.curdir + os.sep]
-  if os.altsep is not None:
-    current_dir_prefixes.append(os.curdir + os.altsep)
-  for prefix in current_dir_prefixes:
-    if filename.startswith(prefix):
-      return filename[len(prefix):]
-
-
 def _RemoveTestSuffix(filename):
   base, _ = os.path.splitext(filename)
   suffixes = [ '_test', '_unittest', '_browsertest' ]
@@ -103,8 +94,6 @@ def _ObjCppImplementation(filename):
 
 
 def _CreateFile(filename):
-  filename = _RemoveCurrentDirectoryPrefix(filename)
-
   contents = _GetHeader(filename) + '\n'
 
   if filename.endswith('.h'):