build: Remove powermock

Incompatible with mockito 4.x, and a bad practice anyway

Signed-off-by: Álvaro Brey <alvaro.brey@nextcloud.com>

app/build.gradle

@@ -152,7 +152,6 @@ ext {
     okhttpVersion = "4.9.3"
     materialDialogsVersion = "3.3.0"
     parcelerVersion = "1.1.13"
-    powermockVersion = "2.0.9"
     retrofit2Version = "2.9.0"
     workVersion = "2.7.1"
     markwonVersion = "4.6.2"
@@ -295,9 +294,6 @@ dependencies {
 
     testImplementation 'junit:junit:4.13.2'
     testImplementation 'org.mockito:mockito-core:4.5.1'
-    testImplementation "org.powermock:powermock-core:${powermockVersion}"
-    testImplementation "org.powermock:powermock-module-junit4:${powermockVersion}"
-    testImplementation "org.powermock:powermock-api-mockito2:${powermockVersion}"
 
     androidTestImplementation "androidx.test:core:1.4.0"