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

libexec/pyenv-rehash

@@ -1,4 +1,3 @@
-#!/usr/bin/env bash
 # Summary: Rehash pyenv shims (run this after installing executables)
 
 set -e
@@ -62,7 +61,6 @@ fi
 # serves as a locking mechanism.
 create_prototype_shim() {
   cat > "$PROTOTYPE_SHIM_PATH" <<SH
-#!/usr/bin/env bash
 set -e
 [ -n "\$PYENV_DEBUG" ] && set -x