Remove unused Jetty context files

We used to not do auto deployment in jetty and have explicit context files (see https://github.com/xwiki/xwiki-platform/commit/7b287ea3837c63a8cb6ca90e2d4398d423f446d0#diff-5e6c676b2255617ee0bdfed4063e75a8a8a4cf6ce0e0f6683ccf2d50d2e37374R57-R59 ). This was changed in https://github.com/xwiki/xwiki-platform/commit/aa4bc1bb047ada96287e4f12bc4ab8865e7e46f6#diff-5e6c676b2255617ee0bdfed4063e75a8a8a4cf6ce0e0f6683ccf2d50d2e37374L55-R73 but the context files, useless now, were still left over.