You should be able to make the field empty to remove the Bandwidth limit. According to your screenshot, that does not work for you. I just tested in my lab with firmware version 6.4.4.8-4.2.4.8_60300, and there it worked fine.
What firmware version are you running (can be found in Maintenance menu)?
If you can upgrade firmware to see if that resolves the issue, please try so. If that is not an option you might try removing the command via the CLI (ssh/console port). If you check the configuration, the bandwidth limit will be in the wlan access rule:
wlan access-rule MySSID
index 11
rule any any match any any any permit
bandwidth-limit downstream 222
bandwidth-limit upstream 333
Then go into config mode and remove the bandwidth-limit commands:
IAP-Lab# configure t
We now support CLI commit model, please type "commit apply" for configuration to take effect.
IAP-Lab (config) # wlan access-rule MySSID
IAP-Lab (Access Rule "MySSID") # no bandwidth-limit downstream
IAP-Lab (Access Rule "MySSID") # no bandwidth-limit upstream
IAP-Lab (Access Rule "MySSID") # exit
IAP-Lab (config) # exit
IAP-Lab# commit apply
committing configuration...
configuration committed.
And the configuration should be gone.