lang/elixir-devel: remove hard dependency on erlang24

Since moving from OTP21 as default OTP in lang/erlang, to OTP24,
we no longer have incompatible bytecode versions between default
erlang and newer builds. Thus, there is no reason to hard-wire
the dependency when a simple PATH will work as expected.

Remove broken SRC as build option.

PR: 264046
PR: 257235
Reported by: Pat Maddox <pat@patmaddox.com>
Sponsored by: SkunkWerks, GmbH