Remove deprecated fields

[benashz]

In preparation for the major version bump, all deprecated fields related to < vault-1.2 have been removed.

As this PR is quite large, all related updates to the upgrade guide will be done in a separate PR.

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Release note for CHANGELOG:

TBD

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccXXX'
=== RUN   TestDataSourceIdentityEntityName
--- PASS: TestDataSourceIdentityEntityName (2.18s)
=== RUN   TestDataSourceIdentityEntityAlias
--- PASS: TestDataSourceIdentityEntityAlias (2.04s)
=== RUN   TestDataSourceIdentityGroupName
--- PASS: TestDataSourceIdentityGroupName (2.05s)
=== RUN   TestDataSourceIdentityGroupAlias
--- PASS: TestDataSourceIdentityGroupAlias (2.10s)
[...]
--- PASS: TestResourceToken_basic (2.45s)
=== RUN   TestResourceToken_import
--- PASS: TestResourceToken_import (2.78s)
=== RUN   TestResourceToken_full
--- PASS: TestResourceToken_full (2.63s)
=== RUN   TestResourceToken_lookup
--- PASS: TestResourceToken_lookup (2.65s)
=== RUN   TestResourceToken_expire
--- PASS: TestResourceToken_expire (16.30s)
=== RUN   TestResourceToken_renew
--- PASS: TestResourceToken_renew (27.59s)
=== RUN   TestAccTransitCacheConfig
--- PASS: TestAccTransitCacheConfig (7.09s)
=== RUN   TestTransitSecretBackendKey_basic
--- PASS: TestTransitSecretBackendKey_basic (3.86s)
=== RUN   TestTransitSecretBackendKey_rsa4096
--- PASS: TestTransitSecretBackendKey_rsa4096 (6.67s)
=== RUN   TestTransitSecretBackendKey_import
--- PASS: TestTransitSecretBackendKey_import (2.85s)
=== RUN   TestExpandAuthMethodTune
--- PASS: TestExpandAuthMethodTune (0.00s)
=== RUN   TestFlattenAuthMethodTune
--- PASS: TestFlattenAuthMethodTune (0.00s)
=== RUN   TestValidateNoTrailingSlash
--- PASS: TestValidateNoTrailingSlash (0.00s)
PASS
ok  	github.com/hashicorp/terraform-provider-vault/vault	710.069s

CircleCI received exit code 0
...

[tomhjp]

Code changes look good to me, although I think I'm missing some context on why a couple of the fields are being deleted.

One question on the upgrade experience: if users update deprecated fields to use the new field name, will the terraform plan report any diff?

[tomhjp]

Discussed the upgrade path offline and not much we can do to avoid diff noise. LGTM!