Fix warning about unused |EVP_PKEY_METHOD.ctrl_str|.

Some compilers in some configurations warn about this structure member
not being assigned a value. Since it is never used anywhere, just
remove it.

Change-Id: I46064234961bf449fe5fcb88594ddb3ff390e7d7
Reviewed-on: https://boringssl-review.googlesource.com/5621
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>