Update assq-delete-all to support keys of string type

[Stephen J. Turnbull]

Kaushal writes:

 > I wanted to use assq-delete-all to remove all elements in an alist whose
 > car matched a string.

Aside: "assq" means to use the test `eq' when comparing keys.  You
would name this function `assoc-delete-all' if you just want to permit
strings as well as symbols and numbers (as the suggestion to use
cl-remove does).

 > (defun assq-delete-all-string (key-str alist)

In this version, where you restrict to string, I would name it
`string-assoc-delete-all' or `assoc-string-delete-all'.

I don't know if it makes sense to perpetuate the traditional names
with the availabilty of cl-lib.