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

pyenv.d/exec/pip-rehash/conda

@@ -1,5 +1,3 @@
-#!/usr/bin/env bash
-
 set -e
 [ -n "$PYENV_DEBUG" ] && set -x
 

pyenv.d/exec/pip-rehash/easy_install

@@ -1,5 +1,3 @@
-#!/usr/bin/env bash
-
 set -e
 [ -n "$PYENV_DEBUG" ] && set -x
 

pyenv.d/exec/pip-rehash/pip

@@ -1,5 +1,3 @@
-#!/usr/bin/env bash
-
 set -e
 [ -n "$PYENV_DEBUG" ] && set -x