drop detection code for Cyrix CPUs older than the Cyrix M2

The 486DLC is a 486-class CPU which we no longer support on i386.

The 6x86 (also known as the M1) did not support CPUID by default[*]
so extra support code is needed to differentiate between these early
Cyrix processors in order to apply some errata that the M1 needs.
However the 6x86 doesn't implement the RDTSC instruction so we can
remove support code for this CPU at this point.

Cyrix implemented RDTSC in the 6x86MX (also known as the M2). So
this is likely the earliest Cyrix CPU that we can support on i386.

We keep the support code in "cyrix6x86_cpu_setup" because early
6x86MX CPUs would continue to benefit from this fixup code.

[*]: CPUID can be enabled on the Cyrix 6x86 by setting bit 7 of
CCR4

ok mlarkin@, jsg@

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