hs-v3: Remove the circuit_established intro flag

Only use the HS circuit map to know if an introduction circuit is established
or not. No need for a flag to keep state of something we already have in the
circuit map. Furthermore, the circuit map gets cleaned up properly so it will
always have the "latest truth".

This commit also removes a unit test that was testing specifically that flag
but now we rely solely on the HS circuit map which is also tested few lines
below the removed test.

Fixes #32094

Signed-off-by: David Goulet <dgoulet@torproject.org>