DROP MEASUREMENT drops from all retention policies

__Steps to reproduce:__
List the minimal actions needed to reproduce the behavior.

1. Create a database
2. Create 2 retention policies
3. Write one value to the same measurement name in both policies
4. Try to delete the measurement from one RP and not the other
* `DROP MEASUREMENT rp1.cpu`

__Expected behavior:__
Measurement and its data is only deleted from one retention policy

__Actual behavior:__
Parsing the query returns an error or, if the RP is left off of the statement, the measurement and its data is deleted from all retention policies.

Start by reproducing the two-retention-policy scenario from the issue and compare `DROP MEASUREMENT rp1.cpu` with an unqualified statement. Trace the query parsing and deletion path; done means the qualified form succeeds and removes data only from the selected retention policy, while the unqualified behavior is explicitly preserved or corrected according to the project’s intended semantics.

Written by the indexing model from the issue text.