Disable built-in ledger cache and browser tools when using IndyScan as the ledger browser

[WadeBarnes]

Support for linking to an IndyScan instance for ledger browsing was recently added, bypassing the build-in ledger browsing tools. However, in the background, the ledger cache is still maintained and updated, and the built-in browser tools are still available.

The cache and build-in browser tools should be disabled when using IndyScan as the ledger browser. i.e. When INDY_SCAN_URL is set. This will save on memory and disk usage.