TS-1996: disable deprecation warnings for the rfc5861 plugin

The RFC5861 plugin still uses the deprecated TSHttpTxnNewCacheLookupDo()
API. Turn off -Wdeprecated until this plugin can use a different
API.
TS-1996: turn off rfc5861 warnings in CFLAGS too
Revert "TS-1996: turn off rfc5861 warnings in CFLAGS too"

This reverts commit 7a41b0fde531edf919490f9a4b17e51a1a6b6dec.
Revert "TS-1996: disable deprecation warnings for the rfc5861 plugin"

This reverts commit f1b5270034e3582a0e7cda9e1ba75929920f7a46.
Revert "TS-1996: Apply TS_DEPRECATED compiler annotation"

This reverts commit a955b08020c8179ca6997747f7bdf35b1204d251.