[dev.boringcrypto] cmd/compile: remove the awful boringcrypto kludge
    
    CL 60271 introduced this “AwfulBoringCryptoKludge.”
    iant approved that CL saying “As long as it stays out of master...”
    
    Now that the rsa and ecdsa code uses boring.Cache, the
    “boring unsafe.Pointer” fields are gone from the key structs, and this
    code is no longer needed. So delete it.
    
    With the kludge deleted, we are one step closer to being able to merge
    dev.boringcrypto into master.
    
    For #51940.
    
    Change-Id: Ie549db14b0b699c306dded2a2163f18f31d45530
    Reviewed-on: https://go-review.googlesource.com/c/go/+/395884
    Reviewed-by: Ian Lance Taylor <iant@golang.org>
    Reviewed-by: Ian Lance Taylor <iant@google.com>