[KeyVault] Fix generated reference documentation for KeyVaultClient

[schaabs]

• Renamed CustomKeyVaultClient to KeyVaultClient to fix 'alias' documentation
• updated azure.keyvault.rst to pull in inherited members
• removed unneeded / outdated rst files

[azuresdkci]

Can one of the admins verify this patch?

[codecov-io]

Codecov Report

Merging #2619 into keyvault_1.0_preview will not change coverage.
The diff coverage is 100%.

@@                  Coverage Diff                  @@
##           keyvault_1.0_preview    #2619   +/-   ##
=====================================================
  Coverage                  56.3%    56.3%           
=====================================================
  Files                      6530     6530           
  Lines                    141715   141715           
=====================================================
  Hits                      79786    79786           
  Misses                    61929    61929

Impacted Files	Coverage Δ
...keyvault/azure/keyvault/custom/key_vault_client.py	96.66% <100%> (ø)	⬆️
azure-keyvault/azure/keyvault/__init__.py	100% <100%> (ø)	⬆️

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[lmazuel]

@schaabs should we rebase this on master now?