compute_sha2: drop support for OS X Leopard

When computing the SHA-2 checksum, `openssl dgst` is a potential candidate if
openssl was found in PATH. Previously, openssl from Homebrew would also get
explicitly added to the candidates list because, historically, Homebrew did not
link it to PATH, OS X Leopard did not have `shasum`, and system openssl could
not calculate sha256 checksums.

This drops support for OS X Leopard which last had an update 15 years ago.

Ref. b396ad7

bin/ruby-build

@@ -313,9 +313,7 @@ compute_sha2() {
     output="$(shasum -a 256 -b)" || return 1
     echo "${output% *}"
   elif type openssl &>/dev/null; then
-    local openssl
-    openssl="$(command -v "$(brew --prefix openssl 2>/dev/null || true)"/bin/openssl openssl | head -1)"
-    output="$("$openssl" dgst -sha256 2>/dev/null)" || return 1
+    output="$(openssl dgst -sha256 2>/dev/null)" || return 1
     echo "${output##* }"
   elif type sha256sum &>/dev/null; then
     output="$(sha256sum -b)" || return 1