Remove merge_params.force_remove_source_branch

[philhug]

Fixes failed webhooks if remove source branch is enabled on branch.
Closes: #300

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[ankitm123]

Why is this removed? Should it be changed instead to

ForceRemoveSourceBranch bool `json:"force_remove_source_branch"`

and verified that gitlab sends boolean instead of string (also fix testdata)?

[zzhzero]

force_remove_source_branch type is inconsistent

[garethjevans]

Closing this in favour of #301