rm useless `if(ax._mainLinePosition)` condition
rm useless `if(ax._mainLinePosition)` condition

- ax._mainLinePosition appears to be always defined for cartesian axes

aburato/plotly.js,aburato/plotly.js,plotly/plotly.js,etpinard/plotly.js,asolagmbh/plotly.js,plotly/plotly.js,plotly/plotly.js,etpinard/plotly.js,asolagmbh/plotly.js,etpinard/plotly.js,asolagmbh/plotly.js,plotly/plotly.js