wic: partition: simlify calling plugin methods

Replaced parse_sourceparams function with list comprehension.
Used local variables instead of attributes.
Moved global variable to the local scope.

[YOCTO #10619]

(From OE-Core rev: 4adbac84046ff744f1452b5ff4d017d17d2d45e2)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/wic/utils/oe/misc.py

@@ -222,26 +222,3 @@ def get_bitbake_var(var, image=None, cache=True):
     get_var method of BB_VARS singleton.
     """
     return BB_VARS.get_var(var, image, cache)
-
-def parse_sourceparams(sourceparams):
-    """
-    Split sourceparams string of the form key1=val1[,key2=val2,...]
-    into a dict.  Also accepts valueless keys i.e. without =.
-
-    Returns dict of param key/val pairs (note that val may be None).
-    """
-    params_dict = {}
-
-    params = sourceparams.split(',')
-    if params:
-        for par in params:
-            if not par:
-                continue
-            if not '=' in par:
-                key = par
-                val = None
-            else:
-                key, val = par.split('=')
-            params_dict[key] = val
-
-    return params_dict