Refactoring: Use bindSigTyVarsFV in rnMethodBinds

`rnMethodBinds` was explicitly using `xoptM` to determine if
`ScopedTypeVariables` is enabled before bringing type variables
bound by the class/instance header into scope. However, this `xoptM`
logic is already performed by the `bindSigTyVarsFV` function. This
patch uses `bindSigTyVarsFV` in `rnMethodBinds` to reduce the number
of places where we need to consult if `ScopedTypeVariables` is on.

This is purely refactoring, and there should be no user-visible
change in behavior.
24 jobs for master in 43 minutes and 36 seconds (queued for 3 seconds)
Status Job ID Name Coverage
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Name Stage Failure
failed
lint-submods Lint
Entering 'libraries/unix'
Entering 'libraries/xhtml'
Entering 'nofib'
Entering 'utils/haddock'
Entering 'utils/hsc2hs'
Authenticating with credentials from job payload (GitLab Registry)
$ git fetch "$CI_MERGE_REQUEST_PROJECT_URL" $CI_MERGE_REQUEST_TARGET_BRANCH_NAME
fatal: No path specified. See 'man git-pull' for valid url syntax
ERROR: Job failed: exit code 1
failed
validate-x86_64-linux-deb9-hadrian Build
hadrian/build-cabal --flavour=validate -j4 --broken-test= binary-dist failed
Running after script...
$ .gitlab/ci.sh clean
Running make --quiet clean...
Running rm -Rf _build...
Uploading artifacts...
WARNING: junit.xml: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1