Remove TruffleRuby from CI

There is currently a bug in TruffleRuby ([truffleruby#3285][]) wherein
`Kernel#binding` does not capture a given anonymous block, which breaks
`yield` calls evaluated using that binding.  This bug is preventing us
from extending our rendering API.

Furthermore, this bug was caught by a test unit for said API extension,
but our views themselves aren't tested by CI.  So if such a `yield` call
were added to a view, CI would still pass but the `sdoc` command would
be broken when using TruffleRuby.

Also, Rails itself does not officially support TruffleRuby.

Considering all of the above, this commit removes TruffleRuby from CI.

[truffleruby#3285]: oracle/truffleruby#3285

.github/workflows/test.yml

@@ -18,7 +18,6 @@ jobs:
           - '3.1'
           - '3.2'
           - 'ruby-head'
-          - 'truffleruby-head'
 
     steps:
       - uses: actions/checkout@v1