BOA: rm Approach.LastUS

[garymm]

it is always equal to US when it is checked

also minor simplificaiton of if / else in Approach.Action

Change-Id: I0a5bf75ad3a0c0f734ecb45ce2c7b44cacd61ba8

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.16 :warning:

Comparison is base (31cc3f3) 30.62% compared to head (a0ef91f) 30.46%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #257 +/- ## ========================================== - Coverage 30.62% 30.46% -0.16% ========================================== Files 79 79 Lines 15157 15153 -4 ========================================== - Hits 4642 4617 -25 - Misses 10256 10270 +14 - Partials 259 266 +7 ``` | [Impacted Files](https://app.codecov.io/gh/emer/axon/pull/257?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=emer) | Coverage Δ | | |---|---|---| | [examples/boa/approach\_env.go](https://app.codecov.io/gh/emer/axon/pull/257?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=emer#diff-ZXhhbXBsZXMvYm9hL2FwcHJvYWNoX2Vudi5nbw==) | `89.22% <100.00%> (-1.88%)` | :arrow_down: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/emer/axon/pull/257/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=emer)

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[garymm]

Hmm this might not be right. Investigating.