CLEANUP: log-format: useless file and line in json converter

CLEANUP: log-format: useless file and line in json converter

The caller must log location information, so this information is
provided two times in the log line. The error log is like this:

   [ALERT] 327/011513 (14291) : parsing [o3.conf:38]: 'http-response
   set-header': Sample fetch <method,json(rrr)> failed with : invalid
   args in conv method 'json' : Unexpected input code type at file
   'o3.conf', line 38. Allowed value are 'ascii', 'utf8', 'utf8s',
   'utf8p' and 'utf8ps'.

This patch removes the second location indication, the the same error
becomes:

   [ALERT] 327/011637 (14367) : parsing [o3.conf:38]: 'http-response
   set-header': Sample fetch <method,json(rrr)> failed with : invalid
   args in conv method 'json' : Unexpected input code type. Allowed
   value are 'ascii', 'utf8', 'utf8s', 'utf8p' and 'utf8ps'.