[Gardening] Remove DoubleWidth related dead code

[palimondo]

Remove auxiliary benchmark and tests for DoubleWidth which was removed from stdlib and stashed into prototype in #15470. These scattered remains were just a dead code. I'm leaving a comment breadcrumb in test/Prototypes/DoubleWidth.swift.gyb pointing here, to assist in easy resurrection (revert these commits).

[palimondo]

@swift-ci please benchmark

[palimondo]

@swift-ci please test

[palimondo]

@eeckstein I'd like to remove the DoubleWidthDivision benchmark that always returns zero because the tested type was removed from stdlib. Please review 🙏

[swift-ci]

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Improvement
UTF8Decode_InitDecoding	228	200	-12.3%	1.14x (?)
UTF8Decode_InitFromBytes_ascii	311	289	-7.1%	1.08x (?)
Removed
DoubleWidthDivision	0	0	0	—

Performance: -Osize

TEST	MIN	MAX	MEAN	MAX_RSS
Removed
DoubleWidthDivision	0	0	0	—

Performance: -Onone

TEST	OLD	NEW	DELTA	RATIO
Regression
String.data.Empty	91	98	+7.7%	0.93x (?)
Improvement
UTF8Decode_InitDecoding_ascii	428	393	-8.2%	1.09x (?)
Removed
DoubleWidthDivision	0	0	0	—

How to read the data

The tables contain differences in performance which are larger than 8% and differences in code size which are larger than 1%.

If you see any unexpected regressions, you should consider fixing the
regressions before you merge the PR.

Noise: Sometimes the performance results (not code size!) contain false
alarms. Unexpected regressions which are marked with '(?)' are probably noise.
If you see regressions which you cannot explain you can try to run the
benchmarks again. If regressions still show up, please consult with the
performance team (@eeckstein).

Hardware Overview

  Model Name: Mac Pro
  Model Identifier: MacPro6,1
  Processor Name: 12-Core Intel Xeon E5
  Processor Speed: 2.7 GHz
  Number of Processors: 1
  Total Number of Cores: 12
  L2 Cache (per Core): 256 KB
  L3 Cache: 30 MB
  Memory: 64 GB