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.

tests/x-mkd.bats

@@ -1,14 +0,0 @@
-#!/usr/bin/env bats
-
-@test "x-mkd creates directory" {
-  dir="$BATS_TMPDIR/mkd-test"
-  run env VERBOSE=1 bash local/bin/x-mkd "$dir"
-  [ "$status" -eq 0 ]
-  [ -d "$dir" ]
-}
-
-@test "x-mkd with no args shows usage" {
-  run bash local/bin/x-mkd
-  [ "$status" -eq 1 ]
-  [[ "$output" == "Usage:"* ]]
-}