chore: remove unused x-mkd and x-validate-sha256sum.sh scripts

x-mkd's cd-in-subshell cannot work when executed (only sourced) and
is unused in the repo. x-validate-sha256sum.sh duplicates the
functionality of x-sha256sum-matcher.

local/bin/x-validate-sha256sum.sh

@@ -1,68 +0,0 @@
-#!/usr/bin/env bash
-#
-# This script contains a helper for sha256 validating your downloads
-#
-# Source: https://gist.github.com/onnimonni/b49779ebc96216771a6be3de46449fa1
-# Author: Onni Hakala
-# License: MIT
-#
-# Updated by Ismo Vuorinen <https://github.com/ivuorinen> 2022
-##
-
-set -euo pipefail
-
-# Stop program and give error message
-# $1 - error message (string)
-error()
-{
-  echo "(!) ERROR: $1" >&2
-  exit 1
-}
-
-# Check for sha256sum command
-if ! command -v sha256sum &> /dev/null; then
-  error "sha256sum could not be found, please install it first"
-fi
-
-# Return sha256sum for file
-# $1 - filename (string)
-get_sha256sum()
-{
-  sha256sum "$1" | head -c 64
-}
-
-# Validate input arguments
-validate_inputs()
-{
-  if [ -z "${filename:-}" ]; then
-    error "You need to provide filename as the first parameter"
-  fi
-
-  if [ -z "${file_hash:-}" ]; then
-    error "You need to provide sha256sum as the second parameter"
-  fi
-}
-
-# Main validation logic
-validate_file()
-{
-  if [ ! -f "$filename" ]; then
-    error "File $filename doesn't exist"
-  elif [ "$(get_sha256sum "$filename")" = "$file_hash" ]; then
-    echo "(*) Success: $filename matches provided sha256sum"
-  else
-    error "$filename doesn't match provided sha256sum"
-  fi
-}
-
-# Main function
-main()
-{
-  filename=$1
-  file_hash=$2
-
-  validate_inputs
-  validate_file
-}
-
-main "$@"