Remove RSA instructions

[elisabethpatterson]

RSA will not be in use after 30 September. RSA instructions need to be removed at that time.

[chanwilson]

i suggest rsa_token.rst remain for the time being, with the contents being replaced with a link to yubikey_token.rst

The following files mention RSA and will need to be adjusted:

./source/accounts/accounts_and_projects.rst
./source/accounts/index.rst
./source/accounts/role_accounts.rst
./source/accounts/rsa_token.rst
./source/connecting/index.rst
./source/data/globus_example.rst
./source/data/globus_online_data_transfer.rst
./source/data/transferring_data.rst
./source/FAQ/index.rst
./source/software/ssh_clients/Tectia/index.rst
./source/software/x2go.rst
./source/systems/cloud_user_guide.rst
./source/systems/mercury_user_guide.rst
./source/systems/ppan_user_guide.rst