You can use this code:
import arcpy
fc_in = r"C:\Forum\test.gdb\Polyline_chord"
fc_out = r"C:\Forum\test.gdb\Polyline_straight"
sr = arcpy.Describe(fc_in).spatialReference
lines = []
with arcpy.da.SearchCursor(fc_in, ("SHAPE@")) as curs:
for row in curs:
polyline_in = row[0]
polyline_out = arcpy.Polyline(arcpy.Array([polyline_in.firstPoint, polyline_in.lastPoint]), sr)
lines.append(polyline_out)
arcpy.CopyFeatures_management(lines, fc_out)
Kind regards, Xander