Revert "HTTP/2: fixed overlapping memcpy in CONTINUATION frames"

Revert "HTTP/2: fixed overlapping memcpy in CONTINUATION frames"

This reverts commit 2d71bdcf8b34d9a44608920e34f2d031b3a938b6.

This is actually unnecessary. Thanks to Valentin Bartenev for the
analysis

1. ngx_http_v2_handle_continuation() handles split HPACK integers at
   frame boundaries
2. HPACK integers are max 4 bytes (NGX_HTTP_V2_INT_OCTETS)
3. The function is only called when length < 4 (i.e., 1-3 bytes remain)
4. The copy destination is offset by 9 bytes (frame header size)

Since 3 < 9, source and destination never overlap.