[lld/tests] Stop setting the "asserts" and "debug" features

[thakis]

Summary

The last use of REQUIRES: asserts was removed in 2015 in 251b0e268 when the
old COFF linker was removed.

The last use of REQUIRES: debug was removed in 2013 in 72c5d3d7c in favor of
REQUIRES: asserts.

lld's test suite currently has no behavior difference with respect to
assertions or debug builds (and hasn't had it for 6 years). Let's keep it that
way :)

[MaskRay]

Looks great!

lld/test/lit.cfg.py

 if platform.system() in ['FreeBSD', 'NetBSD', 'Linux']:
     config.available_features.add('system-linker-elf')
 
 # Set if host-cxxabi's demangler can handle target's symbols.
 if platform.system() not in ['Windows']:
     config.available_features.add('demangler')
 
 llvm_config.feature_config(
-    [('--build-mode', {'DEBUG': 'debug'}),
-     ('--assertion-mode', {'ON': 'asserts'}),
-     ('--targets-built', {'AArch64': 'aarch64',
+    [('--targets-built', {'AArch64': 'aarch64',
                           'AMDGPU': 'amdgpu',
                           'ARM': 'arm',
                           'AVR': 'avr',
                           'Hexagon': 'hexagon',
                           'Mips': 'mips',
                           'MSP430': 'msp430',
                           'PowerPC': 'ppc',
                           'RISCV': 'riscv',