doc: remove old reference from crypto/README.md

The referenced header file does not exist anymore.

Refs: nodejs#43896

src/crypto/README.md

@@ -22,7 +22,6 @@ the various other crypto files and other parts of Node.js:
 * `crypto_util.h` / `crypto_util.cc` (Core crypto definitions)
 * `crypto_common.h` / `crypto_common.cc` (Shared TLS utility functions)
 * `crypto_bio.h` / `crypto_bio.cc` (Custom OpenSSL i/o implementation)
-* `crypto_groups.h` (modp group definitions)
 
 Of these, `crypto_util.h` and `crypto_util.cc` are the most important, as
 they provide the core declarations and utility functions used most extensively