Remove shebang lines from scripts for performance

All scripts in libexec/ (excluding pyenv) are called through pyenv,
therefore the shebang lines are not necessary. On some systems this
provides a measurable increase in performance of the shell prompt.

Related to pyenv/pyenv-virtualenv#259

plugins/python-build/bin/pyenv-install

@@ -1,5 +1,3 @@
-#!/usr/bin/env bash
-#
 # Summary: Install a Python version using python-build
 #
 # Usage: pyenv install [-f] [-kvp] <version>

plugins/python-build/bin/pyenv-uninstall

@@ -1,5 +1,3 @@
-#!/usr/bin/env bash
-#
 # Summary: Uninstall a specific Python version
 #
 # Usage: pyenv uninstall [-f|--force] <version>

plugins/python-build/test/stubs/stub

@@ -1,4 +1,3 @@
-#!/usr/bin/env bash
 set -e
 
 status=0