Cleanup in stat_sina() code, and remove limitation on maxwidth

[clauswilke]

This pull request does two things:

1. It cleans up some code in stat_sina that makes no sense. In two cases, if/else codepaths execute the exact same code, so there's no need for the if/else construct.

2. It removes the artificial limit on the maxwidth parameter. I provide an example below for a situation where maxwidth > 1 is appropriate.

n <- 200
y1 <- rnorm(n, mean = 5, sd = 3.5)
y2 <- rnorm(n, mean = 12, sd = .4)
y3 <- rnorm(n, mean = 3.5, sd = 2)

df <- data.frame(y = c(y1, y2, y3), x = rep(c("A", "B", "C"), each = n))

ggplot(df, aes(x, y)) + stat_sina(maxwidth = 1.5)

[thomasp85]

Looks good - thanks