win: default shellxescape, shellxquote to empty

Calling cmd.exe in Windows follows a very different pattern from Vim.
The primary difference is that Vim does a nested call to cmd.exe, e.g.
the following call in Vim

    system('echo a 2>&1')

spawns the following processes

    "C:\Program Files (x86)\Vim\vim80\vimrun" -s C:\Windows\system32\cmd.exe /c (echo a 2^>^&1
        ^>C:\Users\dummy\AppData\Local\Temp\VIoC169.tmp 2^>^&1)
    C:\Windows\system32\cmd.exe /c C:\Windows\system32\cmd.exe /c (echo a 2^>^&1
        ^>C:\Users\dummy\AppData\Local\Temp\VIo3C6C.tmp 2^>^&1)
    C:\Windows\system32\cmd.exe  /c (echo a 2>&1
        >C:\Users\dummy\AppData\Local\Temp\VIo3C6C.tmp 2>&1)

The escaping with ^ is needed because cmd.exe calls itself and needs to
preserve the special metacharacters for the last call. However in nvim
no nested call is made, system('') spawns a single cmd.exe process.
Setting shellxescape to "" disables escaping with ^.

The previous default for shellxquote=( wrapped any command in
parenthesis, in Vim this is more meaningful due to the use of tempfiles
to store the output and redirection (also see &shellquote). There is
a slight benefit in having the default be empty because some expressions
that run in console will not run within parens e.g. due to unbalanced
double quotes

    system('echo "a b')

src/nvim/options.lua

@@ -2077,11 +2077,7 @@ return {
       secure=true,
       vi_def=true,
       varname='p_sxq',
-      defaults={
-        condition='WIN32',
-        if_true={vi="("},
-        if_false={vi=""}
-      }
+      defaults={if_true={vi=""}}
     },
     {
       full_name='shellxescape', abbreviation='sxe',
@@ -2089,11 +2085,7 @@ return {
       secure=true,
       vi_def=true,
       varname='p_sxe',
-      defaults={
-        condition='WIN32',
-        if_true={vi='"&|<>()@^'},
-        if_false={vi=""}
-      }
+      defaults={if_true={vi=""}}
     },
     {
       full_name='shiftround', abbreviation='sr',