Remove charter PDF (tektoncd#82)

A copy (for historical purposes) of the original PDF can be found within
the sigstore/TSC repository, but the canonical version of the charter
has been converted to Markdown and is viewable at
https://github.com/sigstore/TSC/blob/main/docs/CHARTER.MD

Signed-off-by: Bob Callaway <[email protected]>