remove openssl RAND_bytes from crypto.pyx

was forgotten there and not used any more since a while.

also removed the hint about PyCrypto being an alternative to crypto.pyx,
it was not updated since 2014, looks pretty stale.

borg/crypto.pyx

@@ -1,17 +1,11 @@
-"""A thin OpenSSL wrapper
+"""A thin OpenSSL wrapper"""
 
-This could be replaced by PyCrypto maybe?
-"""
 from libc.stdlib cimport malloc, free
 from cpython.buffer cimport PyBUF_SIMPLE, PyObject_GetBuffer, PyBuffer_Release
 
 API_VERSION = 3
 
 
-cdef extern from "openssl/rand.h":
-    int  RAND_bytes(unsigned char *buf, int num)
-
-
 cdef extern from "openssl/evp.h":
     ctypedef struct EVP_MD:
         pass