Remove non-standard pow10 filter in favor of exp10

[itchyny]

The pow10(3) function is a non-standard glibc extension removed in 2.27. Listing in the manual often confuses users like #2456, #2566. The exp10(3) function was a glibc extension too, but is standardized in TS 18661-4:2015 and expected to be available widely.

[emanuele6]

Maybe we could add a note in the manual that explains that exp10 is equivalent to pow10, but more likely to be implemented?

[wader]

Some reason pow10 can't be an alias for exp10?

[itchyny]

I prefer making implemenetation clean, there are exp/0, exp2/0 so exp10/0 is easy to remember (even if pow/2 exists), another reason is pow10/0 has not been available in many environments, so I don't think it's important to keep compatibility.

[wader]

@itchyny Ok makes sense, i'm fine with removing it

[itchyny]

Thank you.