MINOR: quic: use the same HP encryption helpers for RX and TX

MINOR: quic: use the same HP encryption helpers for RX and TX

Header protection is applied by generating a mask and XORing it with
the header. Removing it requires the same mask and the same XOR, which
restores the original header.

With AES-128-CTR, AES-256-CTR and ChaCha20, encryption and decryption
produce the same mask from the same key and packet sample. This is a
property of these algorithms, not specific to OpenSSL.

Use quic_tls_enc_hp_ctx_init() and quic_tls_hp_encrypt() for RX too,
with the RX key, and remove the redundant decryption helpers.