*_string() enumerator to string functions: remove common prefix

*_string() enumerator to string functions: remove common prefix

Those functions are used for logging and error cause messages. Removing
this prefix makes the output less noisy. The logging statement typically
indicates what field is logged anyway, for example:

    Destroying event message: addr=0x4f6c580, type=EVENT
    Destroying port: addr=0x4f30c20, type=INPUT
    Destroying structure field class object: addr=0x4eb2e30, type=STRUCTURE
    Finalizing error cause: addr=0x4daa6f0, actor-type=UNKNOWN

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ieb0bda9287bafbd06e2aa0f077d5d631213e5803
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1780
Tested-by: jenkins <jenkins@lttng.org>