net: guangruntong: Drop common.mk build rules at Makefile

[Avenger-285714]

When exec "make clean" after "make distclean", here is follow errors:

drivers/net/ethernet/guangruntong/Makefile:30: *** Cannot find common.mk build rules. Stopped.

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关键摘要:

• 移除了对 common.mk 文件存在性的检查，可能会导致未定义的变量错误，如果 common.mk 文件确实不存在。
• 使用了 $(eval) 而不是 eval 函数来避免 SLES 10 SP4 的 Make 错误，这是一个好的做法，但应该确保所有使用 $(eval) 的代码都已经过测试。

是否建议立即修改:

• 是，建议立即修改以避免潜在的构建错误。同时，应该确保所有使用 $(eval) 的代码都已经过测试，以确保其行为与预期一致。