Prebid 7: remove support for legacy FPD

[dgirardi]

Description

Remove support for setConfig('fpd'), config.getLegacyFpd, config.convertAdUnitFpd and related relics of legacy fpd

Related: #8264

[patmmccann]

#7796 for linkage

[dgirardi]

going through adapters for an unrelated issue, I noticed some uses of legacy fpd that are not trivial to remove. For example:

Prebid.js/modules/adrelevantisBidAdapter.js

Lines 138 to 145 in 2c0518f

	let fpdcfg = config.getLegacyFpd(config.getConfig('ortb2'));
	if (fpdcfg && fpdcfg.context) {
	let fdata = {
	keywords: fpdcfg.context.keywords || '',
	category: fpdcfg.context.data.category || ''
	}
	payload.fpd = fdata;
	}

depending on how many more like these there are, it may be better to reduce the scope of this change to removing just the portion that is public in the publisher API.

[patmmccann]

taxonomy committee indicates support to include this in 7