Remove the assembly references of the widget from the site project and delete the widget registration from Sitefinity page toolbox.
To delete the widget from toolbox go to Administration->Settings->Advanced->Toolboxes->Page Controls->Sections
choose the section in which the widget have been registered
locate the widget and delete it using the trash icon, here
is a screenshot.
Alternatively go to App_Data/Sitefinity/Configuration/ToolboxesConfig.config locate the registration entry for the widget and remove it.
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