out_logdna: remove unused store

[DavidKorczynski]

len is assigned but never used. No need for the operations related to this.

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[leonardo-albertovich]

Is it me or logdna_config_destroy doesn't release _hostname? Additionally, there are a few points in logdna_config_create where flb_free is used instead of logdna_config_destroy even after fields that need to be released have been populated.

Also the check on line 313 should be in line 321 because both branches of the conditional allocate memory, the result of flb_uri_encode is not checked and none of the results of the flb_sds_cat calls are checked either.

I'm just mentioning it in case you want to proactively fix those.

[leonardo-albertovich]

I'm approving this as discussed in slack but please don't forget to issue a followup PR to replace those flb_free(ctx) calls with logdna_config_destroy(ctx), otherwise there will be leaks