Remove support for CompactStrings off

[wenshao]

Initially, the option to disable CompactStrings was provided for performance reasons.

However, with some optimizations for UTF8 scenarios and Latin1, disabling CompactStrings no longer has any performance benefits. I propose removing the related code to make it simpler and easier to maintain.

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[ExE-Boss]

Note that there’s currently a bug with CompactStrings and StringBuilder which can result in a UTF‑16 inflated String being created with only Latin1 characters, which I mentioned in:

• 8282429: StringBuilder/StringBuffer.toString() skip compressing for UTF16 strings #7671 (comment)

This introduced a bug where the following now incorrectly returns false with ‑XX:⁠+CompactStrings as it produces an inflated String containing only Latin1 characters:

new StringBuilder()
	// insert Latin1 sub-slice "foo" of UTF-16 string:
	.insert(0, "foo \uF000", 0, 3)
	.toString()
	.equals("foo");

Such a String can cause many weird bugs.

[wenshao]

Note that there’s currently a bug with CompactStrings and StringBuilder which can result in a UTF‑16 inflated String being created with only Latin1 characters, which I mentioned in:

• 8282429: StringBuilder/StringBuffer.toString() skip compressing for UTF16 strings #7671 (comment)

This introduced a bug where the following now incorrectly returns false with ‑XX:⁠+CompactStrings as it produces an inflated String containing only Latin1 characters:

new StringBuilder()
	// insert Latin1 sub-slice "foo" of UTF-16 string:
	.insert(0, "foo \uF000", 0, 3)
	.toString()
	.equals("foo");

Such a String can cause many weird bugs.

this should be another PR, fixed in the insert method