net: buf: Switch from net buf assert to system assert

Remove net bufs own assertion mechanism and use the system assert
instead. This changes the assertion messaged printed from printing
expression to printing the line and file name. This provides more
context as the same expression could be asserted upon multiple times and
would then not provide enough clarity in the message.

This removes the option to enable only net buf assertions.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>