* lisp/gnus/auth-source.el: Silence lexical-binding warnings

* lisp/gnus/auth-source.el: Silence lexical-binding warnings

(auth-source-netrc-use-gpg-tokens): Simplify (symbol-value 'VAR) to
just VAR.
(auth-source-backend-parse): Use make-instance.
(auth-source-search): Remove unused key args.
Remove unused vars `accessor-key' and `backend'.  Avoid `eval'.
(auth-source-search-backends): Use slot names rather than their initarg.
(auth-source-netrc-create):
(auth-source-delete):
(auth-source-secrets-create, auth-source-plstore-search)
(auth-source-macos-keychain-create, auth-source-macos-keychain-search)
(auth-source-plstore-create, auth-source-netrc-search)
(auth-source-netrc-parse): Remove unused key args.
(auth-source-forget+): Simplify the arglist.
(auth-source-macos-keychain-search-items)
(auth-source-token-passphrase-callback-function): Mark unused args.
(auth-source-epa-extract-gpg-token): Remove unused var `plain'.
(pp-escape-newlines): Declare.
(auto-source--symbol-keyword): New function.
(auth-source-plstore-create, auth-source-netrc-create)
(auth-source-netrc-normalize): Use it.
(auth-source-netrc-search): Don't pass :delete to
auth-source-netrc-parse since it doesn't use it.
(auth-source-plstore-create, auth-source-netrc-create): Use plist-get
symbol-value to index in keyword args.
(auth-source-macos-keychain-result-append): Avoid setq.
(auth-source-netrc-create): Remove unused vars `file' and `add'.
(auth-source-user-or-password): Remove unused var `cname'.