Removing padding from the encryption API

* Padding is fundamentally an operation that occurs prior to
  encryption to make plaintext suitable.
* Even though wether it's regarded is dependent on the mode, it
  can be used safely regardless.
* Moving it out of this API makes the API's simpler and more
  composable.
* Moving it out of this API makes it simpler for backends that
  don't work exactly like OpenSSL's EVP.
* Move it out of this API makes it simpler to include padding
  that OpenSSL's EVP API doesn't expose.
diff --git a/docs/primitives/symmetric-encryption.rst b/docs/primitives/symmetric-encryption.rst
index fe074f3..821e8e0 100644
--- a/docs/primitives/symmetric-encryption.rst
+++ b/docs/primitives/symmetric-encryption.rst
@@ -61,9 +61,3 @@
                                         ``block_size`` of the cipher. Do not
                                         reuse an ``initialization_vector`` with
                                         a given ``key``.
-    :param padding: One of the paddings described below.
-
-Paddings
-~~~~~~~~
-
-.. class:: cryptography.primitives.block.padding.PKCS7()