Remove pythonPath option given to python-shell module (#656)

This fixes the bug where PSLab Desktop on Windows would not connect
to the board unless python3 was linked to python with
`mklink python3.exe python.exe` in the Python installation folder.

See also https://github.com/extrabacon/python-shell/blob/f3b64d3307d8dc15eb9c071d8aa774c1e7d5b2d7/index.ts#L94

Author: cynthi8

background_tasks/linker.html

@@ -7,7 +7,7 @@
 	const log = require('electron-log');
 
 	const bridgePath = getScriptPath('bridge.py');
-	const pyshell = new PythonShell(bridgePath,	{ pythonPath: 'python3'	});
+	const pyshell = new PythonShell(bridgePath);
 	log.info('Python spawned:', pyshell && pyshell.command);
 
 	loadBalancer.job(

background_tasks/playback.html

@@ -8,7 +8,7 @@
 
 	// For distribution, we bundle everything into an asar archive.
 	const bridgePath = getScriptPath('bridge.py');
-	const pyshell = new PythonShell(bridgePath,	{ pythonPath: 'python3'	});
+	const pyshell = new PythonShell(bridgePath);
 	log.info('Python spawned:', pyshell && pyshell.command);
 
 	loadBalancer.job(