Remove special casing for old spec test error messages

[fitzgen]

Passive elements used to have a bad expected error message in the spec tests but that's since been fixed.

[yurydelendik]

I rebased the patch. I'm getting locally (and during checks):

thread 'tests::wast_tests::run_proposals_tests' panicked at 'table_init.wast:192: expected "unknown table 0", got "unknown elem segment 0: segment index out of bounds"', src/tests.rs:442:25

[alexcrichton]

(as a heads up this repo is moved to https://github.com/bytecodealliance/wasm-tools now)