Remove support for creating ARM NOREAD sections.

Remove support for creating ARM NOREAD sections.

gas	* config/obj-elf.c (obj_elf_change_section): Remove support for
	ARM NOREAD sections.
	* config/tc-arm.c (arm_elf_section_letter): Delete.
	* config/tc-arm.h (md_elf_section_letter): Delete.
	* doc/c-arm.texi (ARM Section Attribute): Delete section.
	* testsuite/gas/arm/section-execute-only.d: Delete.
	* testsuite/gas/arm/section-execute-only.s: Delete.

ld	* testsuite/ld-arm/arm-elf.exp: Remove ARM NOREAD section tests.
	* testsuite/ld-arm/thumb1-input-section-flag-match.d: Delete.
	* testsuite/ld-arm/thumb1-input-section-flag-match.s: Delete.
	* testsuite/ld-arm/thumb1-noread-not-present-mixing-two-section.d: Delete.
	* testsuite/ld-arm/thumb1-noread-not-present-mixing-two-section.s: Delete.
	* testsuite/ld-arm/thumb1-noread-present-one-section.d: Delete.
	* testsuite/ld-arm/thumb1-noread-present-one-section.s: Delete.
	* testsuite/ld-arm/thumb1-noread-present-two-section.d: Delete.
	* testsuite/ld-arm/thumb1-noread-present-two-section.s: Delete.