Avoid unnecessary CultureInfo.CurrentCulture accesses in Enum

[stephentoub]

Enum can only be backed by primitive numerical types, and using the IConvertible interface implementations to convert to numerical types won't pay any attention to culture, so just as there's no need to pass through the supplied provider, there's no need to access CultureInfo.CurrentCulture.

(As suggested by @GrabYourPitchforks at #38779 (comment).)

[Benchmark]
public int ToInt() => Convert.ToInt32(DayOfWeek.Tuesday);

Method	Toolchain	Mean	Error	StdDev	Ratio	Allocated
ToInt	\master\corerun.exe	20.82 ns	0.108 ns	0.101 ns	1.00	48 B
ToInt	\pr\corerun.exe	16.55 ns	0.247 ns	0.231 ns	0.79	48 B

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