TST: Remove compatibility styling for images that have been updated

If we've updated an image in the past couple commits, then we can remove
the backwards-compatibility styling so that they're generated as
intended.

Author: QuLogic

lib/matplotlib/tests/baseline_images/test_agg_filter/agg_filter_alpha.gif

@@ -7,9 +7,6 @@
 @image_comparison(baseline_images=['agg_filter_alpha'],
                   extensions=['gif', 'png', 'pdf'])
 def test_agg_filter_alpha():
-    # Remove this line when this test image is regenerated.
-    plt.rcParams['pcolormesh.snap'] = False
-
     ax = plt.axes()
     x, y = np.mgrid[0:7, 0:8]
     data = x**2 - y**2

lib/matplotlib/tests/baseline_images/test_agg_filter/agg_filter_alpha.pdf

@@ -1027,9 +1027,6 @@ def test_hexbin_pickable():
 def test_hexbin_log():
     # Issue #1636 (and also test log scaled colorbar)
 
-    # Remove this line when this test image is regenerated.
-    plt.rcParams['pcolormesh.snap'] = False
-
     np.random.seed(19680801)
     n = 100000
     x = np.random.standard_normal(n)
@@ -1515,9 +1512,6 @@ def test_pcolormesh_log_scale(fig_test, fig_ref):
 
 @image_comparison(['pcolormesh_datetime_axis.png'], style='mpl20')
 def test_pcolormesh_datetime_axis():
-    # Remove this line when this test image is regenerated.
-    plt.rcParams['pcolormesh.snap'] = False
-
     fig = plt.figure()
     fig.subplots_adjust(hspace=0.4, top=0.98, bottom=.15)
     base = datetime.datetime(2013, 1, 1)

lib/matplotlib/tests/baseline_images/test_agg_filter/agg_filter_alpha.png

@@ -832,9 +832,6 @@ def _mask_tester(norm_instance, vals):
 
 @image_comparison(['levels_and_colors.png'])
 def test_cmap_and_norm_from_levels_and_colors():
-    # Remove this line when this test image is regenerated.
-    plt.rcParams['pcolormesh.snap'] = False
-
     data = np.linspace(-2, 4, 49).reshape(7, 7)
     levels = [-1, 2, 2.5, 3]
     colors = ['red', 'green', 'blue', 'yellow', 'black']
@@ -852,9 +849,6 @@ def test_cmap_and_norm_from_levels_and_colors():
 @image_comparison(baseline_images=['boundarynorm_and_colorbar'],
                   extensions=['png'], tol=1.0)
 def test_boundarynorm_and_colorbarbase():
-    # Remove this line when this test image is regenerated.
-    plt.rcParams['pcolormesh.snap'] = False
-
     # Make a figure and axes with dimensions as desired.
     fig = plt.figure()
     ax1 = fig.add_axes([0.05, 0.80, 0.9, 0.15])