remove backward compatible flag support

[kruskall]

APM Server version (apm-server version): 9.0

Description of the problem including expected versus actual behavior:

beats removed backward compatible flag support in elastic/beats#42117 but we're adding it back at https://github.com/elastic/apm-server/blob/main/internal/beatcmd/init.go#L57-L61

We should remove the hack as well

Steps to reproduce:

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