MSP430: Remove target-specific handling of the "persistent" attribute

The "persistent" attribute is now handled generically, and does not
need specific support in the MSP430 back end.

gcc/ChangeLog:

	* config/msp430/msp430.c (msp430_section_attr): Don't warn for "lower"
	attribute used with "noinit" or "persistent" attributes.
	(msp430_persist_attr): Remove.
	(attr_lower_exclusions): Remove ATTR_PERSIST exclusion.
	(attr_upper_exclusions): Likewise.
	(attr_either_exclusions): Likewise.
	(attr_persist_exclusions): Remove.
	(msp430_attribute_table): Remove ATTR_PERSIST handling.
	(msp430_handle_generic_attribute): Remove ATTR_PERSIST section conflict
	handling.
	(TARGET_ASM_INIT_SECTIONS): Remove.
	(msp430_init_sections): Remove.
	(msp430_select_section): Use default_elf_select_section for decls with
	the "persistent" attribute.
	(msp430_section_type_flags): Remove ".persistent" section handling.
	* doc/extend.texi (MSP430 Variable Attributes): Remove "noinit" and
	"persistent" documentation.

gcc/testsuite/ChangeLog:

	* g++.target/msp430/data-attributes.C: Remove expected warnings for
	"lower" attribute conflicts.
	Adjust expected wording for "persistent" attribute misuse.
	* gcc.target/msp430/data-attributes-2.c: Likewise.
	* gcc.target/msp430/pr78818-auto-warn.c: Likewise.