Remove SHA1 utility used in doc Makefile

# Makefile for LAMMPS documentation

SHELL         = /bin/bash

SHA1          = $(shell echo ${USER}-${PWD} | python utils/sha1sum.py)

BUILDDIR      = ${PWD}

RSTDIR        = $(BUILDDIR)/rst

VENV          = $(BUILDDIR)/docenv

#!/bin/env python

# simple utility which reimplements sha1sum using Python

import hashlib

import sys

s = hashlib.sha1()

s.update(sys.stdin.read().encode())

print(s.hexdigest())