Remove/Separate benchmarking dependencies

[diamondburned]

I think this module, as a TOML parser, shouldn't depend on another TOML and YML parser for benchmarking.

It would probably be best if another repository could be made for benchmarks that rely on other libraries such as BurntSushi's. Otherwise, I think it's better to remove the benchmarks.

For reference, here's the go.mod as of this issue:

module github.com/pelletier/go-toml

go 1.12

require (
	github.com/BurntSushi/toml v0.3.1 // for benchmarking
	github.com/davecgh/go-spew v1.1.1
	gopkg.in/yaml.v2 v2.3.0 // also for benchmarking
)

[pelletier]

I agree, would be better with no dependency. I'd still like to keep the benchmarks around in this repository. Wonder if we can put them in a sub-folder with their dedicated go.mod file.

[AllenX2018]

Good idea. I will try to move them to a sub-folder like "benchmark" with a dedicated go.mod file