将字符串str中的所有’M’字符删除掉。如”AbMxyzMktMwhileMMMcd”，变为” Abxyzktwhilecd”

该函数修改源串，源串不能是常量。
char* delete_(char* str, char ch)
{
int i = 0;
int j;
while (str[i] != ch && str[i] != '\0')
++i;
j = i;
while (str[i] != '\0')
{
if (str[i] == ch)
{
++i;
continue;
}
str[j++] = str[i++];
}
str[j] = '\0';
return str;
}