hexagon: Drop invalid UAPI header asm/signal.h

This UAPI header contains declarations of internal kernel symbols.
Such declarations are invalid. Normally headers_check.pl would report
an error, but apparently its dependency on CC_CAN_LINK prevent its
execution on hexagon so far.

As the header does not expose any additional UAPI, just make it a
regular internal kernel header. asm-generic/signal.h will be used
for the UAPI automatically, the same as before.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://patch.msgid.link/20251223-uapi-nostdinc-v1-2-d91545d794f7@linutronix.de
Signed-off-by: Nathan Chancellor <nathan@kernel.org>