Remove unused signature features

[Mike-M0]

Describe your changes

• only create xmldsig fragments which are listed (like other fragments)
• remove unused fields for optimization (e.g remove "shall not be used" from standard)
  removed elements are not in structure and will result in en/decoder error NOT_IMPLEMENTED when "isUsed" flag is set or element is accessed in decoder.

Issue ticket number and link

Feature request #75

Checklist before requesting a review

• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• I read the contribution documentation and made sure that my changes meet its requirements

[Mike-M0]

The change is not optimal.
I needed a manual change of eventCode in xmldsigFragment en/decoder from 1 to 32 and bitcount from 1 to 6 (which are the values from before this feature), because all other eventcodes are getting optimized out.
I could not find a easy way to implement this in generator.
But with this manual fix, PnC works fine again.

[barsnick]

Notes for us when reviewing this:

• Can this or should this be configurable (on/off)?
• Does it affect the API?
• Does it affect the ABI?
• Is it free of regressions?

[barsnick]

For the record, from my experiments:

• All the elements disappear from the API.
• Binary code size reduction (x86_64, standard generator options), iso-2 only: 1.7 to 2.1 % (reduced from 390272 to 383568 bytes stripped, 747752 to 731944 bytes unstripped)
• Memory size reduction: from
  ISO-2 DC Struct sizes: exiDoc: 24184, V2G_Message: 24184, Header: 10360, Body: 13824
  to
  ISO-2 DC Struct sizes: exiDoc: 17064, V2G_Message: 17064, Header: 3240, Body: 13824

Our regression tests seem to pass.

[barsnick]

If we take this in, we should also do the same optimizations for DIN and ISO 15118-20.

Applied to DIN and ISO -20:
HEAD @ d0b5310:

SAP Struct sizes:
	exiDoc: 588

DIN Struct sizes:
	exiDoc: 10976
	V2G_Message: 10976
	Header: 7976
	Body: 3000

ISO-2 DC Struct sizes:
	exiDoc: 24184
	V2G_Message: 24184
	Header: 10360
	Body: 13824

ISO-20 CommonMessages Struct sizes:
	exiDoc: 318392

ISO-20 AC Struct sizes:
	exiDoc: 10920

ISO-20 DC Struct sizes:
	exiDoc: 10840

With a modified version of this PR, rebased on HEAD:

SAP Struct sizes:
	exiDoc: 588

DIN Struct sizes:
	exiDoc: 4320
	V2G_Message: 4320
	Header: 1316
	Body: 3000

ISO-2 DC Struct sizes:
	exiDoc: 17064
	V2G_Message: 17064
	Header: 3240
	Body: 13824

ISO-20 CommonMessages Struct sizes:
	exiDoc: 311280

ISO-20 AC Struct sizes:
	exiDoc: 6000

ISO-20 DC Struct sizes:
	exiDoc: 6000