libkvm: Remove a fallback for pre-5.0 i386 kernels

[jhb]

Summary

5.0 exported a kernbase symbol with the value of KERNBASE for use in
libkvm (commit f85f3040062dffca6dfcb0c3c2ac64d99cc19db3). This
fallback allowed libkvm to still examine crashdumps from kernels
without that change on i386. However, the value of KERNBASE for i386
hasn't actually been static, e.g. it changed during the 4G/4G change,
so this fallback hasn't worked for several years anyway.