Removed unused configuration from the README

[tanaka-takayoshi]

I received a support case that some parameters are no longer available.
README says maxBufferSize and maxRecords are deprecated.

https://github.com/newrelic/newrelic-fluent-bit-output

newrelic-fluent-bit-output/config/config.go

Lines 121 to 125 in 747dc5c

	func checkDeprecatedConfigFields(ctx unsafe.Pointer) {
	checkDeprecatedConfigField(ctx, "maxBufferSize")
	checkDeprecatedConfigField(ctx, "maxRecords")
	checkDeprecatedConfigField(ctx, "maxTimeBetweenFlushes")
	}

However, as far as I reviewed this repo, these parameters are not used now.