fscrypt: remove CRYPTO_CTR dependency

fscrypt doesn't use the CTR mode of operation for anything, so there's
no need to select CRYPTO_CTR.  It was added by commit 71dea01e
("ext4 crypto: require CONFIG_CRYPTO_CTR if ext4 encryption is
enabled").  But, I've been unable to identify the arm64 crypto bug it
was supposedly working around.

I suspect the issue was seen only on some old Android device kernel
(circa 3.10?).  So if the fix wasn't mistaken, the real bug is probably
already fixed.  Or maybe it was actually a bug in a non-upstream crypto
driver.

So, remove the dependency.  If it turns out there's actually still a
bug, we'll fix it properly.

Signed-off-by: Eric Biggers <ebiggers@google.com>