Remove dependency on DataValues.jl

[svilupp]

Closes https://github.com/JuliaPlots/StatsPlots.jl/issues/534

All tests pass locally. I have not found any impacted functionality.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (f1dc52d) 26.34% compared to head (4aa59f4) 26.34%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #535 +/- ## ======================================= Coverage 26.34% 26.34% ======================================= Files 20 20 Lines 1241 1241 ======================================= Hits 327 327 Misses 914 914 ``` | [Impacted Files](https://codecov.io/gh/JuliaPlots/StatsPlots.jl/pull/535?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots) | Coverage Δ | | |---|---|---| | [src/StatsPlots.jl](https://codecov.io/gh/JuliaPlots/StatsPlots.jl/pull/535?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaPlots#diff-c3JjL1N0YXRzUGxvdHMuamw=) | `33.33% <ø> (ø)` | |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.

[sethaxen]

Makes sense to me. #87 introduced this dependency because it was already a dependency of IterableTables, which this package no longer depends on, so I suspect this can be removed. @mkborregaard can you think of a reason we need the DataValues dep?

[mkborregaard]

No, I think it makes sense to remove it.