refactor: remove unneeded check

[alexandear]

omittableType == nil is always true, because omittableType is declared as var omittableType types.Type and is nil (types.Type is an interface). This was found by the govet.nilness check:

❯ golangci-lint run
plugin/modelgen/models.go:454:20: nilness: tautological condition: nil == nil (govet)
                if omittableType == nil {
                                 ^

I have:

• Updated any relevant documentation (see docs)

[coveralls]

coverage: 59.74% (-0.01%) from 59.753%
when pulling a57e6f1 on alexandear:refactor/fix-nilness
into 0d5b25e on 99designs:master.