tracing/probes: Remove WARN_ON_ONCE from parse_btf_arg

tracing/probes: Remove WARN_ON_ONCE from parse_btf_arg

Sashiko found that user can cause this WARN_ON_ONCE() easily
with adding a kprobe event based on a raw address with BTF
parameter.

Since this is not an unexpected condition, remove the
WARN_ON_ONCE().

Link: https://lore.kernel.org/all/178177265367.2059927.13789953014706792126.stgit@mhiramat.tok.corp.google.com/
Link: https://sashiko.dev/#/patchset/178165816303.269421.7302603996990753309.stgit%40devnote2
Reported-by: Sashiko <sashiko-bot@kernel.org>
Fixes: b576e09701c7 ("tracing/probes: Support function parameters if BTF is available")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>