delete legacy apm_oss plugin

[pgayvallet]

Summary

Related to #71927

Delete the legacy apm_oss plugin.

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: fe9ae1c

Build metrics

✅ unchanged

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@elasticmachine merge upstream

[pgayvallet]

@elastic/apm-ui following discussion here #71927 (comment), it seems that the remains of the legacy apm_oss plugin were unused and can safely be removed. CI seems to agree with me, but could you confirm that?

[sorenlouv]

We only use apm_oss to declare kibana configs. It also looks like they are also defined in:

kibana/src/plugins/apm_oss/server/index.ts

Lines 26 to 33 in b2a473d

	enabled: schema.boolean({ defaultValue: true }),
	transactionIndices: schema.string({ defaultValue: 'apm-*' }),
	spanIndices: schema.string({ defaultValue: 'apm-*' }),
	errorIndices: schema.string({ defaultValue: 'apm-*' }),
	metricsIndices: schema.string({ defaultValue: 'apm-*' }),
	sourcemapIndices: schema.string({ defaultValue: 'apm-*' }),
	onboardingIndices: schema.string({ defaultValue: 'apm-*' }),
	indexPattern: schema.string({ defaultValue: 'apm-*' }),

So lgtm. Thanks for taking care of this 👍