Inline the MultiStatementObservationHolderFactory dependency

[adrianclay]

Why

This factory didn't appear to be providing any value, and is used only by the ObservationMapper, so inline it to simplify.

Type of change

Internal change (non-breaking change with no effect on the functionality affecting end users)

Checklist:

• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the Changelog with details of my change in the UNRELEASED section if this change will affect end users

[github-actions]

Looks good. All 2 mutations in this change were killed.

class	surviving	killed
💯uk.nhs.adaptors.gp2gp.ehr.mapper.diagnosticreport.ObservationMapper	0	2

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