Remove only overlapping ticks in subplots grid

I have created a subplots grid without any spaces between the subplots, with shared x,y-axes. I only show the ticks and labels for the outer subplots. The problem is that the tick numbers overlap at the borders of the subplots. With MaxNLocator, I can remove the upper or lower ticks, but only for all plots at once.

Question: How can I keep the highest tick only for certain plots (in this case x=2.0 only in the bottom right subplot, and y=3 only in the top left subplot)? Why does my conditional setting of ticks for certain subplots fail?

Code:

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.ticker import MaxNLocator

numPlotsY = 3
numPlotsX = 3
f, ax_grid = plt.subplots(numPlotsY,numPlotsX,sharex=True,sharey=True)

A = np.arange(numPlotsY)+1.0
phi = np.arange(numPlotsX)
x = np.linspace(0,2.0,100)
fontsize = 18

for y_i in range(0,numPlotsY):
    for x_i in range(0,numPlotsX):
        y = A[y_i]*np.sin(x*np.pi + phi[x_i])
        ax = ax_grid[y_i,x_i]
        ax.plot(x,y,lw=2.0)

        if x_i == 0:
            ax.set_ylabel(r'$y$', fontsize=fontsize)

        if y_i == numPlotsY-1:
            ###########################
            # Why doesn't this work?! #
            ###########################
            if x_i != numPlotsX-1:
                nbins = len(ax.get_xticklabels())
                ax.xaxis.set_major_locator(MaxNLocator(nbins=nbins,prune='upper'))
            else:
                nbins = len(ax.get_xticklabels())
                ax.xaxis.set_major_locator(MaxNLocator(nbins=nbins,prune=None))
            ax.set_xlabel(r'$x/pi$', fontsize=fontsize)

        if y_i == 0:
            ax.set_title(r'$phi=%s$' % phi[x_i], fontsize=fontsize)

        if x_i == numPlotsX-1:
            ax.annotate(r'$A=%d$' % A[x_i], xy=(1.1,0.5), rotation=90,
                        ha='center',va='center',xycoords='axes fraction', fontsize=fontsize)

f.subplots_adjust(wspace=0,hspace=0)
plt.suptitle(r'$Acdotsinleft(2pi x + phiright)$',fontsize=18)
plt.show()